1. Field of the Invention
This invention relates to thermoplastic polyurethane (TPU)/polyolefin (PO) blends. In one aspect the invention relates to such blends comprising a polyphosphate-type intumescent flame retardant while in another aspect, the invention relates to such blends further comprising a liquid phosphate modifier.
2. Description of the Related Art
Polyolefin polymers are widely used in the construction of wire and cable coverings, e.g., semiconductive shields, insulation layers, protective jackets, etc. due to their relatively low cost and good physical and chemical properties. Notwithstanding their generally good mechanical properties, better mechanical properties are desired and by replacing some of the polyolefin content with TPU, the mechanical properties are significantly improved, e.g., elongations greater than (>) 400% and tensile strengths >1200 psi versus 100% elongation and less than (<) 1000 psi tensile strengths with the polyolefin only. In addition, the compositions incorporating the TPU/polyolefin blend component show improved flexibility and higher deformation temperature, which are needed in certain wire and cable applications.
Also, the TPU/polyolefin blends show improved tape extrusion performance versus a comparable TPU-only composition, indicating improved melt rheology/fabrication capability for extrusion applications such as wire and cable use. By blending in low cost polyolefins with the TPU, the cost of the plastic is significantly reduced allowing new formulation latitude for both TPU and polyolefin end uses. The TPU/polyolefin blends have good utility as the base resin system for halogen-free, flame retardant formulations, particularly phosphorus-based, intumescent types.
However, halogen-free flame retardant compositions using the TPU/polyolefin base resins typically show poor strain whitening and scratch whitening performance. The challenge is to improve the strain and scratch whitening performance of these flame retardant compositions without any significant trade-offs in the overall performance balance.